The aim of the present invention is a process for improving the stability and/or preventing the deactivation of the catalyst in processes of manufacture of acetic acid and/or of methyl acetate, as well as a complete process of manufacture of acetic acid and/or of methyl acetate comprising this process.
More specifically, the aim of the present invention is an improved process which enables improving the stability and/or preventing the deactivation of the catalyst in the cases of the processes of manufacture of acetic acid and/or of methyl acetate by isomerisation of methyl formate and optionally by carbonylation of methanol, in the presence of a catalytic system comprising at least one halogenated promoter and at least one iridium-based compound.
The invention also relates to the particular conditions under which this stabilisation process is applied in a particularly advantageous manner.
Various means of access to acetic acid and/or to methyl acetate are known and made use of industrially. Amongst these figures the reaction of carbonylation of methanol which is carried out in liquid phase, under pressure of carbon monoxide which is one of the reagents, in the presence of a homogenous catalytic system. Another means of access to acetic acid consists in carrying out the isomerisation of methyl formate. This reaction is, itself, generally carried out in the presence of a catalytic system in a homogenous phase. Finally, according to another process, the carbonylation of methanol and the isomerisation of methyl formate are carried out simultaneously.
The process of carbonylation with rhodium is a known process, which is made use of industrially, and which has been the subject of numerous articles and patents, such as, for example, American patents U.S. Pat. Nos. 3,769,329 and 3,813,428.
European patents EP 618 183 and EP 618 184, as well as European patents EP 785 919 and EP 759 022, describe a process of carbonylation in the presence of a catalytic system which is based on iridium and which, if need be, further contains rhodium.
A process of carbonylation with iridium and ruthenium, which is currently made use of industrially, is described in European patent EP 643 034.
More recently, a new means of access constituted by a methyl formate isomerisation reaction in the presence of iridium has been proposed in French patent FR 2,746,794 and International application WO 97/35829.
In parallel, a process of preparation of acetic acid and/or of methyl acetate is proposed in the patent FR 2,746,795 and the International application WO 97/35828, said process simultaneously makes use of a methyl formate isomerisation reaction and a methanol carbonylation reaction.
These various processes of production of acetic acid are generally carried out continuously in installations which essentially comprise three zones. The first corresponds to the reaction zone per se, which comprises a reactor under pressure in which the carbonylation and/or the isomerisation are carried out in liquid phase. The second is constituted by a zone for separating the acid formed. This operation is carried out by partial vaporisation of the reaction mixture in an apparatus called a flash apparatus wherein the pressure is maintained lower than in the reactor. The vaporised part is then sent into a third zone wherein the acetic acid is purified. This zone comprises, for example, various distillation columns in which the acetic acid produced is separated from the water, the reagents and the by-products. The part of the mixture which remains in the form of liquid leaving the vaporisation zone, and which notably comprises the catalyst, is recycled to the reactor.
It is known to the person skilled in the art that the second zone of the installations of production of acetic acid and/or of methyl acetate is generally the site of a deactivation and/or of a precipitation of the catalyst, and this whatever the process employed, amongst the processes described supra, be.
These phenomena are generally caused by the low carbon monoxide pressure which prevails in this zone, and these phenomena are accentuated by low water contents. In U.S. Pat. No. 5,237,097, a proposed solution consists in introducing carbon monoxide into the liquid supply to the flash, in order to maintain a sufficient partial pressure of said component in the flash.
French patent FR 2,726,556 and International application WO 96/14286 describe a process of injection of carbon monoxide into the liquid fraction originating from the flash, in order to reactivate the catalyst recycled to the reactor.
European patent EP 0 616 997 and the corresponding European Divisional Application EP 0 786 447 propose an improvement of the process of manufacture of acetic acid by carbonylation catalysed with iridium, this improvement consisting in maintaining the water content at higher than 0.5% by weight in the liquid fraction originating from the flash, in order to stabilise the catalyst present in this fraction.
The prior art does not propose any improvement in relation to the stabilisation of the catalyst in processes of manufacture of acetic acid and/or of methyl acetate by isomerisation of methyl formate and optionally processes of carbonylation of methanol catalysed with iridium.
The inventors of the present invention have now discovered in an entirely surprising way that the problem of the deactivation and of the destabilisation of the catalyst could be solved by the upkeep of a sufficient overall content of formic acid and of methyl formate in the liquid fraction originating from the flash and, this, even in the presence of a particularly low water content in this part of the installation, and even for water contents of less than 0.5% by weight with respect to the non-vaporised liquid fraction, and even in the almost absence of carbon monoxide in the medium although, up to present, the person skilled in the art considered that a relatively high water content, and in any case greater than 0.5%, was necessary in order to ensure the stability of the catalyst, and that a minimal carbon monoxide content enabled the catalyst to be stabilised.
Hence, the invention relates, according to a first object, to an improvement which can be applied to the processes of manufacture of acetic acid and/or of methyl acetate which make use of an iridium-based catalytic system, this improvement being intended for improving the stability and/or preventing the deactivation of the catalyst.
This stabilisation process enables lowering the water content considerably, both in the reaction medium and in the flash zone, and this constitutes a non-negligible advantage from an economical point of view, since it enables limiting costs in the additional step of recovery of the product formed. This stabilisation of the catalyst has thus enabled defining the conditions of an improved process of preparation of acetic acid and/or of methyl acetate which includes the improvement intended to stabilise the catalyst and according to which, furthermore, a particularly reduced water content is used.
Hence, according to a second aspect, the invention relates to a complete process of manufacture of acetic acid and/or of methyl acetate, under satisfactory conditions, both as regards the stabilisation of the catalyst, and the water concentration, and this constitutes a double advantage from an economical point of view with respect to the processes which exist at present.
More specifically, according to the first aspect supra, the invention relates to a process for improving the stability and/or preventing the deactivation of the catalyst in processes of manufacture of acetic acid and/or of methyl acetate according to which processes, in a first step, referred to as the reaction step, at least one methyl formate isomerisation reaction is carried out in liquid phase, in the presence of carbon monoxide and of a catalytic system comprising at least one halogenated promoter and at least one iridium-based catalytic compound, and, in a second step, referred to as the flash step, the partial vaporisation of the reaction medium originating from the first step is carried out in a separator referred to as the flash separator. Such a process consists in maintaining an overall content of formic acid and of methyl formate at least equal to 1% by weight of said liquid fraction, preferably between 1 and 50%, preferably between 1 and 30%, by weight with respect to said liquid fraction, in the non-vaporised liquid fraction originating from said flash separator.
The invention also relates, according to the second aspect supra, a complete process of manufacture of acetic acid and/or of methyl acetate comprising a first step, referred to as the reaction step, during which at least one methyl formate isomerisation reaction is carried out, in liquid phase, in the presence of carbon monoxide and of a catalytic system comprising at least one halogenated promoter and at least one iridium-based catalytic compound, and a second step, referred to as the flash step of partial vaporisation of the reaction medium originating from the first step, is carried out in a separator referred to as the flash separator. In accordance with this process, an overall content of formic acid and of methyl formate at least equal to 1% by weight of said liquid fraction is maintained in the non-vaporised liquid fraction originating from said flash separator.
According to this process, the water content in the liquid fraction originating from the flash will advantageously be maintained at less than 5% by weight, preferably less than 2%, and more preferably less than 0.5% by weight with respect to said liquid fraction originating from the flash.
The process of stabilisation and of maintenance of the activity of the catalyst set forth supra, as well as the complete process of manufacture of acetic acid and/or of methyl acetate with regard to one or the other processes of manufacture of acetic acid and/or of methyl acetate in which the reaction which is carried out in the reaction step necessarily comprises a liquid phase reaction of isomerisation of methyl formate in the presence of carbon monoxide and of a catalytic system comprising at least one halogenated promoter and at least one iridium-based catalytic compound.
However, according to an advantageous variant of each of the two aspects of the invention, this methyl formate isomerisation reaction is carried out simultaneously with a methanol carbonylation reaction, and it is possible for this carbonylation of methanol to be demonstrated by the consumption of the carbon monoxide introduced in the reaction step.
According to another advantageous variant, the process of stabilisation and of maintenance of the activity of the catalyst of the present invention is effected by controlling the water content in the liquid fraction originating from the flash. This content is advantageously maintained at less than 5% by weight and preferably less than 2% by weight with respect to said liquid fraction originating from the flash.
It is even possible, as set forth supra, to obtain entirely advantageous results of the stability of the catalyst by maintaining water contents at less than 0.5% by weight in the liquid fraction originating from the flash. This constitutes, as set forth supra, a considerable advantage over the processes of the prior art.
According to a particularly advantageous variant of the invention, the process of manufacture of acetic acid and/or of methyl acetate makes use, in addition to the first step referred to as the reaction step and the second step referred to as the flash step, of a third step referred to as the step of purification and recovery of the acetic acid and/or of the methyl acetate from the vaporised fraction originating from the partial vaporisation step.
During this step, the acetic acid and/or the methyl acetate are separated from the light compounds such as water, formic acid, by various means known to the person skilled in the art.
According to a particularly advantageous variant of the invention, the formic acid is separated from the acetic acid by reactive distillation by injecting methanol into the lower part of the distilling column, and by removing the purified acetic acid at the bottom of the column and the methanol and methyl formate mixture at the head of the column.
In the description that follows, attention will be paid to defining particularly advantageous conditions both in the reaction medium and in the liquid medium originating from the flash, conditions which apply both to the process of stabilisation and/or of maintenance of the activity of the catalyst, and to the complete process of manufacture of the acetic acid and/or of the methyl acetate.
In the description that follows and unless indicated otherwise, the term  less than  less than reaction greater than  greater than  is understood as meaning the whole of the reactions taking place in the reaction zone, it being of course that this notion covers reactions of isomerisation and optionally of carbonylation, as well as every equilibrium, which exist in the reaction zone.
Thus, in particular, the term  less than  less than reaction temperature greater than  greater than  is understood as meaning the temperature imposed during the reaction step.
In general, the reaction is carried out at a temperature between 150 and 250xc2x0 C. More particularly, the reaction temperature is between 175 and 210xc2x0 C. Preferably, it is between 175 and 200xc2x0 C.
The total pressure under which the reaction is generally conducted is greater than atmospheric pressure. More particularly, it is advantageously less than 200.105 Pa and, preferably, less than or equal to 50.105 Pa. The pressures are expressed in absolute Pascals, and are measured in the hot, i.e. under the conditions of reaction temperature.
The partial carbon monoxide pressure is preferably maintained between 0.5.105 and 15.105 Pa.
The overall content of formic acid and of methyl formate is, in the reaction medium, advantageously maintained at a value at least equal to 1% by weight of the reaction mixture, preferably between 1% and 50% and preferably between 1% and 30%.
The catalytic system will now be described.
All the iridium compounds which are soluble or able to be dissolved in the reaction medium, under the conditions of implementation of the invention, may be used. As examples, and without intending to be limiting, metallic iridium, its simple salts, its oxides or even its co-ordination complexes may notably be appropriate in the implementation of the invention.
As simple iridium salts, the iridium halides are conventionally used. The halogen is most particularly selected from chlorine, bromine and iodine, the latter being preferred. Thus, compounds such as IrI3, IrBr3, IrCl3, IrI3.4H2O, IrI4, IrBr3.4H2O may be used in the process according to the invention.
Oxides selected from IrO2, Ir2O3.xH2O may equally be conveniently used in the process according to the invention.
As regards the soluble co-ordination complexes of iridium, the compounds which are most commonly used are those having ligands selected from carbon monoxide, or a carbon monoxide/halogen combination, the halogen being selected from chlorine, bromine or more particularly iodine. It is not nevertheless excluded to use soluble iridium complexes whose ligands are selected from the organo-phosphorus compounds and organo-nitrogen compounds for example.
As co-ordination complexes known to the person skilled in the art which are particularly convenient in the implementation of the invention, Ir4(CO)12, Ir(CO)2I2xe2x88x92Q+, Ir(CO)2Br2xe2x88x92Q+, Ir(CO)2Cl2xe2x88x92Q+, may be cited without intention to limit; in which formulae Q may be notably hydrogen, an NR4 group, or a PR4 group, with R selected from hydrogen or a hydrocarbon radical.
These catalysts may be obtained by any process known to the person skilled in the art. Thus, the EP 657 386 and EP 737 103 patents may be referred to for the preparation of iridium-based catalytic solutions which are appropriate for the implementation of the present invention.
It is to be noted that the reaction according to the invention may be carried out with a catalytic system which comprises an iridium compound alone but also, further, rhodium compounds.
The compounds based on iridium and rhodium are described in the patent EP 0 618 183.
When a catalytic system is needed which contains rhodium, it will be possible for the atomic ratio of rhodium to iridium to vary within wide limits between 0.01 and 99.
Generally, the concentration of iridium or, if need be, of (iridium+rhodium) in the reaction medium is between 0.1 and 100 mmol/l, preferably between 1 and 20 mmol/l.
The addition of a catalyst selected from the metals of Group VIII of the periodic classification of the elements can be made with the compounds of iridium or iridium+rhodium mixtures.
In addition to the compounds mentioned supra, the catalytic system according to the invention comprises a halogenated promoter. This halogenated promoter can be in the form of a halogen alone, or in combination with other elements such as, for example, hydrogen, a methyl radical or an acetyl radical.
The halogen is in general selected from chlorine, bromine or iodine, iodine being preferred.
As halogenated compounds which can also be used as promoters, iodine, hydroiodic acid, methyl iodide and acetyl iodide can be cited.
Preferably, methyl iodide will be used as halogenated promoter.
According to a variant of the invention, the halogenated promoter is introduced into the reaction medium, partially or totally, in the form of a precursor. In such a case, said precursor is generally in the form of a compound which can release, into the reaction medium, the hydrocarbon radical of the halogenated promoter mentioned above, under the action of a halogen or notably of the hydrohalic acid, the latter compounds being present in the medium or even introduced to this end.
As non-limiting examples of suitable precursors, compounds selected from methanol, dimethyl ether, methyl acetate or methyl formate, can be cited, which are used alone or in a mixture.
The amount of halogenated promoter present in the reaction mixture is advantageously less than or equal to 20%, with respect to the total weight of said mixture. Preferably, the content of halogenated promoter is less than or equal to 15%.
It is to be noted that if the promoter mentioned above is introduced partially or totally, in the form of a precursor, the amount of precursor or of promoter/precursor mixture is such that it enables obtaining an amount which is equivalent to that mentioned above.
In addition to these compounds, the reaction medium contains water, formic acid, methyl formate, methyl acetate and acetic acid, in preferred weight proportions defined infra, which are preferably maintained simultaneously.
The water content is preferably less than 5% by weight with respect to the reaction medium, preferably less than 2%.
The formic acid content is preferably less than 15% by weight of the reaction medium, preferably less than 12%.
The methyl formate content is preferably maintained at less than 20% by weight of the medium with respect to the reaction medium.
According to a particular mode of the invention, the methyl acetate content is less than 40% by weight, preferably less than 20%.
The acetic acid content is not less than 25% in the reaction medium.
The process for improving the stability and/or preventing the deactivation of the catalyst and the process of manufacturing of acetic acid and/or methyl acetate according to the invention can be implemented in the presence of iodides in a form which is soluble in the reaction medium. The iodides can be introduced per se into the reaction medium, but also in the form of compounds which can form soluble iodides.
 less than  less than Iodides greater than  greater than  is understood as meaning ionic species, i.e. species which comprise neither covalent iodides (notably such as the halogenated promoter) nor hydroiodic acid.
Thus, the iodides introduced into said medium, per se, are selected from mineral or organic iodides.
As mineral iodides, the iodides of alkaline-earth or alkali metals can be cited principally, the latter being preferred. Potassium iodide, lithium iodide and sodium iodide can be cited amongst these.
As organic iodides, the organic compounds comprising at least one organo-phosphorus group and/or at least one organo-nitrogen group, can be cited, which react with the iodine-based compounds to give ionic species which contain this halogen. Tetraphenylphosphonium iodide and N-methyltriethylammonium iodide can be mentioned as examples.
Alkali metal or alkaline-earth metal carboxylates or hydroxides, notably such as lithium acetate, potassium hydroxide and sodium hydroxide, can for example be cited as compounds which can form iodides which are soluble in the reaction medium.
Furthermore, it is to be noted that the iodides can have other origins than those indicated above.
Thus, these compounds can originate from impurities such as alkali metals or alkaline-earth metals, impurities which are present in the starting materials employed for preparing the catalytic solution.
The iodides can even originate from corrosion metals which appear during the reaction.
The process for improving the stability and/or preventing the deactivation of the catalyst and the process of manufacturing of acetic acid and/or methyl acetate according to the invention are preferably, implemented in the presence of corrosion metal content of less than a few hundred ppm, preferably of less than 200 ppm. The corrosion metals are notably iron, nickel, chromium, molybdenum and zirconium. The corrosion metal contents in the reaction medium is maintained by any process known to the person skilled in the art, such as, for example, selective precipitation, liquid-liquid extraction, and passage on ion exchange resins.
The conditions in the flash zone will now be described.
The temperature is advantageously maintained between 80xc2x0 C. and 200xc2x0 C., the total pressure between 0 and 20.105 absolute Pa.
The compounds present in the liquid phase originating from the flash are identical to those which are contained in the reaction medium and described above.
The main characteristic of the invention resides in the maintenance of an overall content of formic acid and of methyl formate at least equal to 1% by weight of the liquid fraction originating from the flash, preferably between 1% and 50%, and preferably between 1% and 30%.
Expressed in weight percentages with respect to the non-vaporised liquid fraction originating from the flash, the proportions of the various constituents which are preferably maintained simultaneously are, advantageously, the following:
the content of halogenated promoter is less than 20%, preferably less than 15%,
the water content is less than 5%, preferably less than 2%, and according to a particularly advantageous embodiment, the stability of the catalyst is even assured when the water content is less than 0.5%,
the content of formic acid is less than 15%, preferably less than 12%,
the methyl formate content is maintained at less than 20%,
according to a particular embodiment of the invention, the methyl acetate content is less than 40%, preferably less than 20%,
the acetic acid content is not less than 25%.
The non-vaporised liquid fraction originating from the flash can contain iodides in the form of ionic compounds which are soluble in said fraction (refer to their description in the reaction medium).
The content of carbon monoxide contained in the flash is non-zero. The carbon monoxide can originate from the reaction medium to be vaporised in the form of dissolved and swept out CO. It can, additionally, be injected directly into the liquid fraction originating from the flash and be recycled to the reactor. In any case, the partial carbon monoxide pressure in the flash zone is less than the partial carbon monoxide pressure maintained in the reaction zone.
In the process of manufacture of acetic acid and/or of methyl acetate described above, these two products contained in the gaseous phase originating from the flash are separated from the light compounds, the water, the formic acid and the other impurities, for example by fractional distillation in one or more distillation columns. Some of these compounds can then be recycled to the reactor.
In a preferred embodiment, the formic acid is separated from the acetic acid by reactive distillation by injecting methanol into the lower part of the distilling column, the methyl formate formed preferentially is recycled to the reactor and the thus-purified acetic acid is removed at the base of the column.
In general, the invention advantageously emphasises a process of manufacture which is carried out continuously.
The Examples which follow are given in a purely illustrative manner and are in no way limiting.